1. Field of the Invention
This invention relates to improvements in coating apparatus wherein two or more coating compositions are simultaneously applied to a substrate, such as, say, a web.
2. Description of the Prior Art
In the prior art, as represented by U.S. Pat. Nos. 2,761,417 and 2,761,791, filed in the names of Russell et al. --the contents of which patents are herein incorporated by reference --it is known to simultaneously coat a moving substrate, for example, a photographic film base with varios coating compositions, by the use of a coating hopper which includes one or more outlets for extruding one or more layers directly into a bead from which the substrate is coated in combination with one or more slide surfaces upon which one or more layers flow before being fed into the bead.
The slide portion of the hopper may include one or more separate exit slots, whereby respective liquid coating compositions may be metered from individual supplies and distributed uniformly across respective inclined slide surfaces. Each of the respective compositions flows by gravity as a layer down its respective inclined surface, whereby the layer becomes smooth and of uniform thickness. Where the hopper includes more than one slide surface, the slike surfaces are arranged so that the layers flow on top of one another. At the end of the lowermost slide surface, i.e., the one adjacent to the extrusion outlet(s), the one or more layers flow on top of a bridge of liquid that is established by liquid coating composition from the extruder portion of the hopper. The bridge of liquid coating composition from the extruder portion spans the generally horizontal gap between an outlet edge of the extruder portion of the coating hopper and the generally upwardly moving substrate. The layer(s) of coating composition from the slide surface(s) thus flow over the bridge, and the substrate, as it is advanced into contact with the bridge, simultaneously picks up all the coating compositions from the extruder and the slide surface(s) and these compositions deposit on the substrate as a composite coating of substantially distinct superimposed layers.
While the apparatus function satisfactorily, various defects can arise in the coated layers of certain products if the bridge is disturbed. For example, a particularly noticeable fault is the appearance of longitudinal striations which render the coated web totally unacceptable as a commercial photographic product. The presence of these longitudinal striations tends to increase rapidly as the web coating speed is increased.
Furthermore, as disclosed in the afore-mentioned patent, and in U.S. Pat. No. 2,681,294,--the contents of which are also incorporated by reference --it is advantageous to create a pressure differential between the exposed surfaces of the bridge, such as by the creation of a vacuum on the trailing surface of the bridge to eliminate excessive vibration and/or rupture of the bridge. The range of vacuum levels which may be used is, of course, limited as, if it is too great, the high pressure differential across the bridge may also cause the bridge to be disturbed and/or ruptured. At times, it may develop that the amount of vacuum being used is insufficient to eliminate the excessive vibration in the bridge, and thus it is desirable to increase the amount of such vacuum. Where an increase in vacuum will itself cause disturbance and/or rupture of the bridge, it is sometimes necessary to change coating conditions, such as by lowering web speed, to make satisfactory coatings.
A further problem with the use of the coating hopper described in the afore-mentioned Russell patents is that particles in the liquid coating composition may adhere to the hopper at the lip edge thereof and cause undesirable streaks to be formed in the coating. An apparatus which reduces or minimizes the deleterious effects of such particles on coatings represents a significant contribution to the state of the art.